Ford F-250 Owner's Manual page 395

Maintenance and Specifications
Do not be concerned if your engine
87
sometimes knocks lightly. However,
if it knocks heavily under most
driving conditions while you are
(R+M)/2 METHOD
using fuel with the recommended
octane rating, see your authorized dealer to prevent any engine damage.
FFV engine (if equipped)
If your vehicle is flex fuel capable, it is designed to use Fuel Ethanol
(Ed75–Ed85), "Regular" unleaded gasoline or any mixture of the two
fuels.
Use of other fuels such as Fuel Methanol may cause powertrain
damage, a loss of vehicle performance, and your warranty may be
invalidated.
It is best not to alternate repeatedly between gasoline and E85. If you do
switch fuels, it is recommended that you add as much fuel as
possible—at least half a tank. Do not add less than five gallons (18.9L)
when refueling. You should drive the vehicle immediately after refueling
for at least 5 miles (8 km) to allow the vehicle to adapt to the change in
ethanol concentration.
If you exclusively use E85 fuel, it is recommended to fill the fuel tank
with regular unleaded gasoline at each scheduled oil change.
Fuel quality
Unleaded gasoline engines
If you experience starting, rough idle or hesitation driveability problems
during a cold start, try a different brand of "Regular" unleaded gasoline.
FFV engines
If you experience starting, rough idle or hesitation driveability problems
during a cold start, try a different brand of E85 fuel. If the driveability
problems continue, fill the vehicle with regular unleaded gasoline and
drive vehicle normally until gasoline is used. See your authorized dealer
if the problem persists.
Do not add aftermarket fuel additive products to your fuel tank. It
should not be necessary to add any aftermarket products to your fuel
tank if you continue to use high quality fuel of the recommended octane
rating. These products have not been approved for your engine and
could cause damage to the fuel system. Repairs to correct the effects of
using an aftermarket product in your fuel may not be covered by your
warranty.
395
2012 F-250/350/450/550 (f23)
Owners Guide, 1st Printing
USA (fus)